An Upper Limit to the Dry Merger Rate at z ~ 0.55

The Astronomical Journal (Impact Factor: 4.02). 01/2010; 139(2):794. DOI: 10.1088/0004-6256/139/2/794
Source: arXiv


We measure the fraction of Luminous Red Galaxies (LRGs) in dynamically close
pairs (with projected separation less than 20 $h^{-1}$ kpc and velocity
difference less than 500 km s$^{-1}$) to estimate the dry merger rate for
galaxies with $-23 < M(r)_{k+e,z=0.2} +5 \log h < -21.5$ and $0.45 < z < 0.65$
in the 2dF-SDSS LRG and QSO (2SLAQ) redshift survey. For galaxies with a
luminosity ratio of $1:4$ or greater we determine a $5\sigma$ upper limit to
the merger fraction of 1.0% and a merger rate of $< 0.8 \times 10^{-5}$
Mpc$^{-3}$ Gyr$^{-1}$ (assuming that all pairs merge on the shortest possible
timescale set by dynamical friction). This is significantly smaller than
predicted by theoretical models and suggests that major dry mergers do not
contribute to the formation of the red sequence at $z < 0.7$.

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Available from: Simon P. Driver, Oct 01, 2015
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    ABSTRACT: We present a comprehensive study of the evolution of Luminous Red Galaxies (LRGs) in the latest and final spectroscopic data release of the Sloan Digital Sky Survey. We test the scenario of passive evolution of LRGs in 0.15<z<0.5, by looking at the evolution of the number and luminosity density of LRGs, as well as of their clustering. A new weighting scheme is introduced that allows us to keep a large number of galaxies in our sample and put stringent constraints on the growth and merging allowed by the data as a function of galaxy luminosity. Introducing additional luminosity-dependent weighting for our clustering analysis allows us to additionally constrain the nature of the mergers. We find that, in the redshift range probed, the population of LRGs grows in luminosity by 1.5-6 % Gyr^-1 depending on their luminosity. This growth is predominantly happening in objects that reside in the lowest-mass haloes probed by this study, and cannot be explained by satellite accretion into massive LRGs, nor by LRG-LRG merging. We find that the evolution of the brightest objects (with a K+e-corrected M_r,0.1 < -22.8) is consistent with that expected from passive evolution. Comment: 16 pages, 10 figures, version accepted for publication in MNRAS. Clarifications and references added, Section 6 revised and expanded, conclusions unchanged
    Monthly Notices of the Royal Astronomical Society 01/2010; DOI:10.1111/j.1365-2966.2010.16630.x · 5.11 Impact Factor
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    ABSTRACT: We present a series of colour evolution models for Luminous Red Galaxies (LRGs) in the 7th spectroscopic data release of the Sloan Digital Sky Survey (SDSS), computed using the full-spectrum fitting code VESPA on high signal-to-noise stacked spectra. The colour-evolution models are computed as a function of colour, luminosity and redshift, and we do not a-priori assume that LRGs constitute a uniform population of galaxies in terms of stellar evolution. By computing star-formation histories from the fossil record, the measured stellar evolution of the galaxies is decoupled from the survey's selection function, which also evolves with redshift. We present these evolutionary models computed using three different sets of Stellar Population Synthesis (SPS) codes. We show that the traditional fiducial model of purely passive stellar evolution of LRGs is broadly correct, but it is not sufficient to explain the full spectral signature. We also find that higher-order corrections to this model are dependent on the SPS used, particularly when calculating the amount of recent star formation. The amount of young stars can be non-negligible in some cases, and has important implications for the interpretation of the number density of LRGs within the selection box as a function of redshift. Dust extinction, however, is more robust to the SPS modelling: extinction increases with decreasing luminosity, increasing redshift, and increasing r-i colour. We are making the colour evolution tracks publicly available at
    Monthly Notices of the Royal Astronomical Society 11/2010; 413(1). DOI:10.1111/j.1365-2966.2010.18148.x · 5.11 Impact Factor
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    ABSTRACT: We analyze 1298 merging galaxies with redshifts up to z=0.7 from the Canada-France-Hawaii Telescope Legacy Survey, taken from the catalog presented in Bridge et al. (2010). By analyzing the internal colors of these systems, we show that so-called wet and dry mergers evolve in different senses, and quantify the space densities of these systems. The local space density of wet mergers is essentially dentical to the local space density of dry mergers. The evolution in the total merger rate is modest out to z ~ 0.7, although the wet and dry populations have different evolutionary trends. At higher redshifts dry mergers make a smaller contribution to the total merging galaxy population, but this is offset by a roughly equivalent increase in the contribution from wet mergers. By comparing the mass density function of early-type galaxies to the corresponding mass density function for merging systems, we show that not all the major mergers with the highest masses (M_stellar > 10^11 M_solar) will end up with the most massive early-type galaxies, unless the merging timescale is dramatically longer than that usually assumed. On the other hand, the usually-assumed merging timescale of ~ 0.5-1 Gyr is quite consistent with the data if we suppose that only less massive early-type galaxies form via mergers. Since low-intermediate mass ellipticals are 10 --100 times more common than their most massive counterparts, the hierarchical explanation for the origin of early-type galaxies may be correct for the vast majority of early-types, even if incorrect for the most massive ones. Comment: 10 pages, 8 figures. Accepted by AJ
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